The present invention relates to an electrical circuit for use in an electrical display device that uses coded electrical signals to produce characters and video effects on an output display.
Raster scan type cathode-ray tube (CRT) display devices produce flicker free displays containing a great deal of information composed of many kinds of characters and images. These devices also produce various video modes, such as normal video (i.e. intensified characters on a dark background), reverse video (i.e. dark characters on an intensified background), or highlighting (i.e. half intensity shading of characters).
Raster scan type CRT displays that are used for displaying alphanumeric information are generally divided into a matrix of M rows and N columns. Each coordinate of the M.times.N display matrix defines a character location on the display and is subdivided into a p.times.q character matrix that defines pixel location on the display. A character read only memory (ROM) stores a plurality of electrical patterns, representative of character information, in p'.times.q' matrices corresponding in size to the p.times.q matrices of the M.times.N display matrix. A display random access memory (RAM) stores charcter-address and character-attribute information corresponding to the coordinates of the M.times.N display matrix in separate memory locations. In the above type display, the character-attribute information defines the various video-modes.
To produce a display on the CRT, the display RAM outputs character-address and video-mode information in synchronization with the raster scan operation of the display device. When addressed by the display RAM, the character ROM outputs a corresponding electrical pattern, representative of character information to the video display circuitry. Concurrently, the video-mode information is coupled to the video display circuitry to enable the appropriate video mode circuits. The output of the video display circuitry is coupled to the Z-axis control grid of the CRT to produce the output display.
Display RAMs and character ROMs are used in raster scan type CRT display devices to reduce circuit cost and memory size. However, limitations are placed on the circuit by the display RAM word size in relation to the total number of characters stored in the character ROM and the number of video modes desired. For example, a display device may use a display RAM that stores data in 8 bit words and a character ROM capable of storing 128 characters. The display device also may provide three video modes, such as normal and reverse video and highlighting. Seven bit character-address information is required to access the 128 characters in the character ROM and 2-bit video mode information is required to enable the three video mode circuits. Since the display RAM stores data in 8 bit words, the character-address and video-mode information must be stored in separate memory locations, thereby increasing the size of the display RAM. This limitation can be overcome by increasing the word size of the display RAM, or decreasing the number of characters stored in the character ROM or decreasing the number of video modes, all of which are undesirable.